The invention relates to control means for a tractor power lift mechanism, and has for its object to provide adjustable mixed height and draft control means. Height control enables an implement hitched to the tractor to be automatically maintained at a predetermined height relative to the tractor by the feed-back to the power lift mechanism of signals proportional to variations in said height. Draft control enables the working depth of the implement in the soil to be automatically varied by the feed-back to the power lift mechanism of signals proportional to variations in the draft force exerted by the tractor on the implement, in order to maintain said force at a predetermined value. Adjustable mixed height and draft control enables the two modes of control to function simultaneously with adjustable proportional effect, the height control mode moderating the effect of the draft control mode on the power lift mechanism with the result that the magnitude of the response to said mechanism to feed-back signals proportional to variations in the draft force is reduced.